Optical signal dispersion compensators can correct for chromatic dispersion in optical fiber and are especially useful for bit rates 10 Gb/s and higher. Furthermore, it is advantageous for the dispersion compensator to have an adjustable amount of dispersion, facilitating system installation. It is also advantageous if the tunable dispersion compensator (TDC) is colorless, i.e., one device can compensate many channels simultaneously or be selectable to compensate any channel in the system.
Previously proposed colorless TDCs include ring resonators[1], the virtually imaged phased array (VIPA)[2], cascaded Mach-Zehnder interferometers (MZIs)[3,4,5], temperature-tuned etalons[6], waveguide grating routers (WGRs) with thermal lenses[7], and bulk gratings with deformable mirrors[8]. The bracketed references[ ] refer to publications listed in the attached Reference list. The cascaded MZI approach is particularly promising since it exhibits low loss, can be made with standard silica waveguides, and can be compact. However, most previous MZI-based TDCs required 8 stages and 17 control voltages in one case[3] and 6 stages with 13 control voltages in two others[4, 5]. This large number of stages and control voltages is expensive and power-consuming to fabricate and operate, especially when compensating 10 Gb/s signals. Because fabrication accuracy cannot guarantee the relative phases of such long path-length differences, every stage of every device must be individually characterized. Also, a large number of stages often results in a high optical loss and a large form factor. Additionally, the more the stages, the more difficult it is to achieve polarization independence.
One previous MZI-based TDCs required only 3 stages and 2 control voltages and also included power monitoring and phase shifters to control power levels.[5A]. That device was designed to compensate 40-Gb/s signals. However, a 10-Gb/s version, because of typical birefringence in planar lightwave circuits, would likely have significant polarization dependence. This is because the path-length differences in the MZIs are 4 times longer for a 10-Gb/s version than a 40-Gb/s version, and thus the 10-Gb/s version is significantly more sensitive to birefringence.
What is desired is a polarization-independent simplified MZI-based TDCs having a reduced number of stages and control voltages.